Acetic acid attacks most reactive metals, turning them into soluble acetates and releasing hydrogen, while noble metals stay intact and stainless steel survives only under limited conditions.
Reactive metals that dissolve
Metals that sit high on the electrochemical series lose electrons to the hydrogen ions in acetic acid. The general reaction is:
- Zn + 2 CH₃COOH → Zn(CH₃COO)₂ + H₂↑
- Mg + 2 CH₃COOH → Mg(CH₃COO)₂ + H₂↑
- Al + 3 CH₃COOH → Al(CH₃COO)₃ + 3⁄2 H₂↑
- Fe (carbon steel) + 2 CH₃COOH → Fe(CH₃COO)₂ + H₂↑
Zinc and magnesium react very quickly, even in dilute solutions. Aluminum’s protective oxide layer dissolves slowly, especially above 60 °C, exposing the metal underneath. Carbon steel corroces at a noticeable rate in 5–10 % acetic acid, producing ferrous acetate.
Metals that resist
Gold, platinum and other noble metals have such low reactivity that acetic acid cannot displace hydrogen from them; no visible reaction occurs.
Copper is a borderline case. In deaerated, dilute acid it shows little change, but when oxygen is present the surface oxidises and a green copper acetate crust forms over weeks.
Stainless steel – conditional resistance
Grade 316 stainless steel, with 2–3 % molybdenum, tolerates dilute, room‑temperature acetic acid because its chromium‑oxide film remains intact. At temperatures above 80 °C or in concentrations above 20 %, the film can be breached, leading to pitting. Grade 304 lacks sufficient molybdenum and pits more readily under the same conditions.
How concentration, water and temperature influence corrosion
Pure glacial acetic acid (≈99.8 %) is less aggressive than a 10 % aqueous solution at room temperature because it contains little water to ionise into H⁺ and CH₃COO⁻. However, acetic acid is hygroscopic; even a small amount of absorbed moisture creates a conductive medium and dramatically speeds up attack.
Temperature follows the usual rule of thumb: a 10 °C rise roughly doubles the reaction rate. Heating a 5 % solution to 100 °C can change a slow surface etch into measurable material loss within hours.
Role of dissolved oxygen
Oxygen acts as a depolariser for metals such as copper and certain alloys. Removing oxygen with deaeration columns can suppress corrosion to near‑zero rates, allowing cheaper materials to be used in closed‑loop systems.
Practical guidelines
- Use zinc‑ or magnesium‑free alloys when handling concentrated acetic acid.
- Prefer 316 stainless or higher‑grade alloys for hot, concentrated streams.
- Keep water content low and control temperature to limit attack on vulnerable metals.
- Consider deaeration if copper or nickel alloys are required.